1. Field of the Invention
This invention relates to a radio angle measurement apparatus which is applicable to radar and other electromagnetic detection systems and which utilizes antenna array system(s) with radiator elements distributed at a spacing larger than one half of the propagation wavelength, and which measures the incident angle of a signal by measuring phase differences between the signals that are received by neighboring elements of the antenna array.
2. Description of the Prior Art
The present applicants are the inventors of the "Radio Angle Measurement Apparatus" disclosed in U.S. Pat. No. 3,787,863. That patent includes a survey of the prior art, and discloses an improved radio angle measurement apparatus with the following features:
(1) The inter-element spacings of the antenna arrays are considerably larger than one half of the propagation wavelength, thus allowing for larger gain and more elaborate characteristics of the radiator elements, as well as permitting a higher angular accuracy due to the fact that a small incident angle produces a large phase difference for a large spacing.
(2) The unambiguous sector is large.
(3) At least two different values of d/.lambda. (where d is the inter-element spacing, .lambda. the propagation wavelength) are used with the antenna array, while the total number of radiator elements is small; high angular accuracy is achieved with a total of three or four radiator elements, the minimum possible number being two.
(4) Detection of multiple directions is possible and data rate is high; spatial scanning of the antenna beam is eliminated.
(5) Consideration is paid to the process of correlating the measured sets of inter-element phase differences to the incident angle of the received signal, and particularly to the role played by the resolution of phase discrimination, with the result that it is possible to construct optional embodiments to maximize the possibility of accurate angle measurement.
(6) The degree of freedom is large in selecting the ratio of d/.lambda. values of antenna arrays, and it is possible to independently select optimal spacing and optimal resolution of phase measurement for every application.
Briefly, the above features of the structure disclosed in U.S. Pat. No. 3,787,863 have been accomplished in the following manner:
(1) A radio signal of incident angle .phi. is received by antenna arrays that exhibit at least two different values of d/.lambda. (where d is the inter-element spacing of the radiator elements, .lambda. the propagation wavelength); the resultant inter-element phase differences, of which there are at least two sets, .PSI..sub.1, .PSI..sub.2, etc., in accordance with the difference in d/.lambda. values, are measured.
(2) The incident angle of the received signal is obtained by a phase-difference-to-angle correlating means which correlates all or a part of the measured sets of said phase differences to the incident angle of the signal, making use of previously memorized angle values derived from either theoretically or experimentally known relationships therebetween.
(3) The domain of the sets of .PSI..sub.1, .PSI..sub.2, etc., to be correlated to a certain value of incident angle (hereinafter referred to as a "phase domain") does not necessarily represent a rectangular or cubic rectangular shape within the (.PSI..sub.1, .PSI..sub.2, . . .) space as expressed by .alpha..ltoreq..PSI..sub.1 .ltoreq..beta., .gamma..ltoreq..PSI..sub.2 .ltoreq..delta., etc. (where .alpha., .beta., .gamma. and .delta. are constants.), but may be made to represent, as a sum of rectangular or cubic rectangular shapes, such a polygonal or polyhedral shape as to fit the shape of error distribution of (.PSI..sub.1, .PSI..sub.2, . . .). By shaping the phase domain in this manner, the possibility of accurate angle measurement is increased.
The invention disclosed in U.S. Pat. No. 3,787,863 which is otherwise very useful in the construction of radio angle measurement apparatus, nevertheless has disadvantages in that the phase-difference-to-angle correlating means is required to memorize the incident angle values that correspond to a considerable number of sets of (.PSI..sub.1, .PSI..sub.2, . . . etc.), thereby resulting in complicated structure of the correlating means.
It is therefore an object of the present invention, while preserving the advantages of the patented radio angle measurement apparatus, to provide radio angle measurement apparatus in which the conversion from the measured phase difference values to the incident angle is performed easier and with less complex apparatus than in the patented apparatus.
More particularly, it is an object of the present invention to provide a radio angle measurement apparatus in which signals representing measured values of inter-element phase differences are electronically processed to generate a new signal representative of the received signal incident angle without requiring a correlation operation and the attendant high capacity memory.
It is another object of the present invention to provide a radio angle measurement apparatus in which the output signal representative of the received signal incident angle varies monotonically with the incident angle.
It is still another object of the present invention to provide a radio angle measurement system in which the signal representative of the received signal incident angle is a series of consecutive numbers in binary, trinary, decimal or other numbering system whose cardinal number can be freely selected.